High vacuum



J. L. FOX

HIGH VACUUM July 5, 1966 2 Sheets-Sheet 1 Filed June 12, 1964 i WM Fig. I

July 5, 1966 2 Sheets-Sheet 2 Filed June 12, 1964 United States Patent 3,252,305 HlGH VACUUM John L. Fox, Needham, Mass, assiguor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts Filed June 12, 1964-, Ser. No. 374,644 14 Claims. (Cl. 230-101) The present invention relates to diffusion pump cold caps of the type originally described in U.S. Patent 2,919,061.

It is the principal object of the invention to provide an improved diffusion pump cold cap with conduction cooling rib for cooling the cap by conductive heat transfer to the cool pump wall.

It is a further object of the invention to provide a cold cap which is adjustable to compensate for loose tolerances in diffusion pump dimensions and tilting vapor jet assemblies.

It is a still further object of the invention to provide an improved diffusion pump, with a cold cap, having high speed and high backstreaming reduction factor compared to prior art diffusion pumps.

The invention accordingly comprises a cold cap with an improved mounting and cooling structure and an improved diffusion pump incorporating such a cold cap and utilizing the conventional pump wall cooling means to cool the cold cap.

The objects of the invention are achieved by the arrangement of conductive ribs extending from the cold cap to the pump wall. A conductive leaf spring is secured to the end of each rib and extends above this end. The end of each rib is relieved by a downwardly opening acute angle so that the leaf spring, when unloaded, extends outwardly of the rib and when loaded presses against the diffusion pump wall to assure good heat transfer despite loose tolerances in the vicinity of the pump inlet. Jack screws are provided to reinforce this pressure and to adjust the centering of the cold cap with respect to the diffusion pump vapor jet assembly.

For a further description of the invention, reference should be had to the following detailed description which explains preferred embodiments of the invention in connection with the accompanying drawings wherein:

FIG. 1 is a partially sectional view of an improved diffusion pump, according to a preferred embodiment.

FIG. 2 is a top view corresponding to FIG. 1;

FIGS. 3 and 3A are partially sectional views of an improved diffusion. pump according to another embodiment.

Referring now to FIG. 1, there is shown a diffusion pump comprising a tubular pump wall in with an inlet opening 12, inlet flange 14 and O-ring 16. It should be noted that there is a natural tendency towards eccentricity of the opening 12 due to manufacturing tolerances and welding of the inlet flange 14 to the wall 10. A pump exit foreline 16 and heater block 18 are located at the lower end of the tube wall. The pump wall and foreline are water cooled via tubes 20.

A vapor jet assembly 22 is arranged inside the pump to discharge hot oil vapor jets into the annular pumping space 24 between the assembly and wall 26). The oil vapor is condensed on the cooled wall and returned to the vapor jet assembly via openings 26 where it is revaporized by a heater 18. The vapor jet assembly comprises a vertical array of nozzles arranged for discharging vapors from within the assembly and a fractionating tube 28 for channeling the purest vapors to the topmost nozzle. The vapor jet assembly is formed of stacked tubes 30 and a hot cap 32 arranged essentially as in the com- "ice mercially available HS and HK diffusion pumps. The vapor jet assembly, including hot cap 32, is restrained from upward movement by a tie-down rod 34 and a nut 35 welded to the hot cap and threaded onto the tie-down rod. The tie-down rod is formed of two portions connected by a spring as shown in FIG. 1. The cold cap assembly is held down via a nut 36, threaded to an extension of the tie-down rod and washer 38.

The cold cap assembly comprises a cold cap 40 arranged around the hot cap 32 as taught in U.S. Patent 2,919,061,to Power et al. That is, the cap 40 is out of contact with, but in closely spaced relation to, hot cap 32. Cold cap 46 extends into the exit path of top nozzle 28 to intercept the over-divergent portion of the top vapor jet. Some means must be provided to cool the cold cap so that it removes energy from the intercepted vapors. The prior art fairly suggests conduction cooling either through a coolant coil tied directly to the cold cap 40 or conduction ribs extending to the cold pump wall 10; thermoelectric coolers; radiant coolers; and other forms of cooling. In accordance with the present invention, the conduction rib cooling method is utilized in the improved form described below.

The cooling is accomplished through three conduction ribs 42 silver soldered to the cold cap 40. Each rib extends toward the cold wall 1%), but terminates just short of the cold wall. A brass leaf spring 44 is silver soldered to the end of each rib and extends upwardly over its associated rib. The area of contact of the springs is in close proximity to turn 21 of the wall cooling coils 20. The end of each rib is relieved by a downwardly opening acute angle A. This arrangement naturally loads the spring 44 against the wall 10 to enhance heat transfer to the wall and tends to center the cold cap 40. Each rib is widest at its inner portion and narrower at its Outer portion to minimize Obstacles to gas conduction in the pump inlet. A brass jack screw 46 is silver soldered to the top of each rib to apply further pressure to leaf springs 44. The pressure is adjusted by rotating nut 48 to move the end of the jack outwardly. The jack screws perform the additional important function of aligning the cold cap 40 with the hot cap 32. This expedient overcomes problems of alignment, arising from inlet eccentricity and tilt of the upper portion of the vapor jet assembly, in reliable and economical manner. The three jack screws are easily adjusted to provide proper alignment.

The cold cap is thermally floated with respect to hot cap 32 and the hot tie-down washer 38 by Pyrex cylinders 5f which serve as spacers. Other thermal insulating materials such as alumina or glasses, all generally referred to herein as ceramics, can be used for cylinders 5h. A spacer sleeve 52 surrounds the upper extension of the tie-down rod to protect the cold cap from vapors. The spacers should be vented to provide ease of pumpout from the volumes enclosed by the spacers. The arrangement has the advantages of economy, simplicity and effective insulation of the cold cap from several adjacent high temper-ature parts. Positive radial adjustment of the cold cap do to assure uniformity of spacing from hot cap 32 is provided by the above-describedjack screw spring loading arrangement.

The structural simplicity and ease of adjustment of the cooling assembly are evident from the drawings. The assembly is easily removed from a pump by loosening the jacks and removing nut 36 and washer 38. When the assembly is reinserted in the pump, the springs 44 are naturally loaded against the pump wall and the jack screws can be adjusted to reinforce the loading and align cold cap 40. The effectiveness of the cooling assembly in limiting backstreaming, without substantially cutting pump speed, is demonstrated by the following examples.

Example 1 A nominal 4-inch diffusion pump was tested for speed, backstreaming and ultimate pressure with no cold cap. The results were 800 liters per second peak speed and .0102 milligram per square centimeter test area per minute backstreaming rate.

The pump was then fitted with a cold cap having a water circulating coil fitted around the cap and water pipes leading from the coil through the pump wall. The cold cap was structurally floated with respect to the hot vapor jet assembly as described above in connection with the cooling assembly of the present invention. The tests were repeated with the results: speed650, backstreaming.000237.

The water cooling coils and cold cap were removed and a cooling assembly, as shown in the drawings, was mounted in the pump. This cooling assembly included three copper ribs inch wide by inch thick minimum cross-section dimensions, brass leafs, inch by inch wide by 1 inches long. The jack screws were made of brass. The Pyrex spacers were inch wall, /2 inch OD. The tests were rerun with the results: speed 720, backstreaming.000267.

Example 2 A similar series of tests was runwith a 6-inch diifusion pump:

- Back- Speed, l./s. streaming, Ing./cm. min.

(a) No cold cap 1, 550 067 (b) Cold cap with water coil 1, 500 00021 (0) Cold cap with conduction ribs, as described- 1, 390 000839 The results of the examples can be recapitulated as follows:

The 18.5% reduction in speed using the water coil in the 4" pump can be attributed to the substantial obstacle presented by water pipes extending from the cold cap to pump wall. The conduction rib arrangement which only had a 10% reduction in peak speed is obviously especially suited to small pumps. However, small pumps present special challenges to the use of an economical conduction structure which will reliably center the floating cold cap. It is apparent from the test results that the principal range of application of conduction cooled caps is in pumps of about IO-inch inlet size and below. A larger pump would preferably use extra ribs with smaller widths and of lesser thickness than the smaller pumps. A total of six ribs would be suitable. A larger pump could be accommodated by three ribs by increasing the contact area between the springs and the pump wall.

The backstrearning reduction factors of 38.2 and 80 obtained in the 4" 6" pump tests using the invention are not attributable to conduction ribs, per se, but to conduction ribs with high conduction leafs spring loaded and jacked against the cold pump wall to insure good heat transfer.

The production costs of the are substantially less than those cooling assembly.

Another embodiment of the invention is shown in FIG. 3. In this embodiment, the cold cap is solely supported by the connections to the pump wall via the ribs, leafs and jack screws (not shown). No connection to the vapor jet assembly 122 is provided. A pointer 102 is threaded through the cold cap to locate a recess in the top of the hot cap 132. Once this location is established, the jack screws are tightened to preserve this location and the pointer is withdrawn as shown in FIG. 3A.

Other variations can be made within the scope of the present invention. For instance, the number of ribs or number of ribs equipped with springs or screws can be varied. The conductive cooling can be used in combination with other modes of cooling such as radiant cooling by blackening the exterior surface of the cold cap. The inclination and shape of ribs can be varied. Each of these variations represents a departure from the optimum combination of cost and performance advantages embodied in the above examples. However, they can be employed without departing from the scope of the principal invention presented herein. It is therefore intended that the description of the preferred embodiment given above and in the accompanying drawings shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. An improved cold cap cooling assembly for mounting over a jet nozzle of a diffusion pump vapor assembly, the vapor jet assembly being mounted within the cooled tubular wall of a diffusion pump comprising, in combination, a cold cap fitting over the said jet nozzle and extending into the exit path of hot vapors emerging from the nozzle, means separating the cold cap from its associated jet nozzle, a plurality of spaced conduction ribs extending from the cold cap substantially to the cooled pump wall, the ribs extending parallel to the tube axis, leaf springs secured to the outer ends of the ribs, 9. face of each leaf butting against the end of its associated rib and extending above the end of the rib, the outer ends of the ribs being relieved so as to substantially form downwardly opening acute angles with respect to the pump wall to thereby spring load the upper portions of the leafs against the wall and means mounted in the assembly for increasing the pressure of the leafs against the wall and positioning the cold cap.

2. A diffusion pump cold cap with a plurality of outwardly extending metal ribs attached to the sides of the cold cap at equally spaced intervals, a leaf spring of high thermal conductivity attached to the outer end of each rib and extending beyond its connection to the rib and force applying means on each rib for varying the outward pressure of the leaf spring connected to the rib.

3. The article of claim 2 wherein the ribs are widest at the inner portion thereof nearest the cold cap and narrowest at the outer portion thereof.

4. The article of claim 2 with the outer ends of the ribs being relieved away from the further extension of the leaf spring.

5. The article of claim 2 wherein the force applying means consist of jack screws mounted on the ribs and pressing against the extended portion of the leaf.

6. A diffusion pump cold cap with a plurality of equally spaced, parallel, planar ribs attached to the cold cap and extending outwardly from the cold cap, a leaf spring attached to the end of each rib and extending laterally above the end of its associated rib, a jack screw attached to the top of each rib and being adjustable to apply pressure to the leaf associated with its supporting rib and adjust the position of the cold cap when the assembly is mounted in a diffusion pump.

present cooling assembly of a water coil type 7. A dilfusion pump comprising a vertically arranged tubular wall with an upper inlet opening, means for cooling the wall, a central vapor jet assembly within the pump having a vertical array of annular, vapor jet nozzles for discharging vapors outwardly, with the topmost nozzle being at the top of the assembly in the region of the tubular inlet, a cold cap assembly for mounting over the topmost nozzle, comprising an annular cold cap member constructed to closely surround the topmost nozzle and extend into the vapor jet emerging from the topmost nozzle, three ribs extending outwardly from the cap to the cooled wall, leaf springs at the ends of the ribs, face portions of the springs contacting the ribs and pump wall and adjustable means mounted on the assembly to force a face portion of each spring against the wall and support the cap in its position surrounding the topmost nozzle without touching said nozzle.

8. The combination of claim 7 wherein the said adjustable means, leaf springs and ribs constitute the sole support of the cold cap.

9. The combination of claim 7 wherein the cold cap is connected to the vapor jet assembly via thermal insulating material to thermally float the cold cap with respect to the hot vapor jet assembly.

10. The combination of claim 9 wherein a tie-down structure extends from the vapor jet assembly, the cold cap having an orifice, the tie-down structure including a rod which passes through said orifice without touching the cold cap when the cold cap and topmost nozzle are aligned, the said thermal insulating material comprising ceramic spacers between the hot cap and topmost nozzle and between the hot cap and tie-down structure.

11. The combination of claim 10 further including a radiation shield between the said orifice in the cold cap and the said rod passing through the orifice.

12. The combination of claim 7 wherein the ribs comprise planar members arranged in vertical planes with the outer ends of the ribs forming acute angles with the pump wall, an inner face of a leaf spring being secured to the end of each rib, each leaf spring extending above the end of its associated rib whereby the extended portion is naturally loaded against the pump wall in face-to-face contact therewith.

13. The combination of claim 12 wherein the adjustable means consists of a jack screw mounted on the top of each planar rib and extending to the said upper port-ion of each spring.

14. The combination of claim 12 wherein the ribs are of stepped form with a wide portion connected. to the cold cap and a narrower outer end portion.

References Cited by the Examiner UNITED STATES PATENTS 5/1950 Morand 230-101 4/1960 Power 230--l0l 

7. A DIFFUSION PUMP COMPRISING A VERTICALLY ARRANGED TUBULAR WALL WITH AN UPPER INLET OPENING, MEANS FOR COOLING THE WALL, A CENTRAL VAPOR JET ASSEMBLY WITHIN THE PUMP HAVING A VERTICAL ARRAY OF ANNULAR, VAPOR JET NOZZLES FOR DISCHARGING VAPORS OUTWARDLY, WITH THE TOPMOST NOZZLE BEING AT THE TOP OF THE ASSEMBLY IN THE REGION OF THE TUBULAR INLET, A COLD CAP ASSEMBLY FOR MOUNTING OVER THE TOPMOST NOZZLE, COMPRISING AN ANNULAR COLD CAP MEMBER CONSTRUCTED TO CLOSELY SURROUND THE TOPMOST NOZZLE AND EXTEND INTO THE VAPOR JET EMERGING FROM THE TOPMOST NOZZLE, THREE RIBS EXTENDING OUTWARDLY FROM THE CAP TO THE COOLED WALL, LEAF SPRINGS AT THE ENDS OF THE RIBS, FACE PORTIONS OF THE SPRINGS CONTACTING THE RIBS AND PUMP WALL AND ADJUSTABLE MEANS MOUNTED ON THE ASSEMBLY TO FORCE A FACE PORTION OF EACH SPRING AGAINST THE WALL AND SUPPORT THE CAP IN ITS POSITION SURROUNDING THE TOPMOST NOZZLE WITHOUT TOUCHING SAID NOZZLE. 